Field of the Invention
The present invention relates to a method for inspecting defects on a mask.
With the constant reduction in minimum structure sizes and the associated, resolution-induced change from optical to non-optical wavelengths during the exposure of semiconductor products, new requirements are also being placed on the minimum structure sizes and on the functional modes of the masks that are needed to structure the wafers. The conventional technique that is used in the exposure step to copy the structure present on the mask in a reduced form or at the same size onto the wafer includes using the optical exposure of the diffraction or the phase properties of the light that is applied during the development of so-called half-tone, alternating and 3-tone phase masks.
During the implementation of these measures on the mask, zones of a third or further type of transparency are structured to achieve higher resolutions on the exposed wafer. By using optical proximity correction (OPC) on the mask, additional artificial structures are also applied that serve merely to compensate for inherent errors in the exposure of the wafer and to bring the exposed structure as close as possible to the data image again.
In recent times, masks for non-optically transferring structures to wafers have also been developed, including those in which radiation is reflected in the extreme ultra-violet range (EUV), and stencil masks, in which the structures are transferred using electron beams or ion beams. Presently, these masks are mostly membrane masks produced from silicon wafers, as opposed to the quartz plates previously used as the basis for masks.
With the increased requirements placed on the lithographic techniques, steps are also needed for defect inspection, which must be managed. First, defects of an even smaller size have to be traced, and second because of the broader range of mask types, it is increasingly also necessary to account for the structural differences between the structure that is applied to the mask and the image achieved on the wafer. As a result of the considerable increases in the speed of computing systems in recent years, changes have increasingly been made from the conventional die-to-die inspection in which identical structures present many times on the mask are compared with one another to die-to-database inspection in which a structure on the mask is compared with a data image stored in the database.
In the case of die-to-database inspection—to make a comparison possible—an algorithm is used to convert the data image from the database into an image that simulates the optical characteristics of the mask transfer. The more complex the optical structure to be transferred, for example, phase-shifted signals or OPC structures, the more time-consuming the inspection becomes, and also the more difficult the development of an underlying algorithm becomes.
If, for conventional chrome-on-glass masks, OPC structures and half-tone phase masks, satisfactory solutions for the algorithms could be delivered within an adequate time interval following the delivery of newly developed types of defect inspection systems, then for the alternating phase masks, the 3-tone face masks, the EUV and stencil masks found in development, the problem arises that the algorithms for the reconstruction of a mask image from its data image will no longer be obtainable in good time. To generate masks with electron-optical or ion-optical characteristics, it is forecast that this effect will emerge to a particularly considerable extent.
One way around the problem would be recourse to die-to-die inspection, which is still being used, but because of the frequently singular structures on the masks, is not always possible. The advantage here would be that the mask image would not have to be simulated. Modern defect inspection systems are designed to store extremely small areas on the mask temporarily in order to then change the coordinates of the reference die, to find the corresponding matching piece of the structure, and to compare the current image with the image temporarily stored.
The same procedure would be possible in the case of mask-to-mask inspection in which the structures of two identical masks are inspected one after another. The image from the first mask is stored temporarily, and during the inspection of the second mask, is compared with the image of the second mask. Unfortunately in this case, the considerable disadvantage arises that retrieving the stored data out of memory is too time-consuming, and that memories in the order of magnitude of some terabytes would be needed which are rarely available at the present time. In addition, it is often the case that necessary interruptions in the inspection cannot readily be carried out.
U.S. Pat. No. 6,043,932 shows an inspection instrument for die-to-die inspection in which, by splitting the laser beam and subsequently adjusting the partial beams onto the individual dies, and by recording the die structures in parallel with a linear image sensor, a comparison of the images is enabled in real time. However, the aforementioned disadvantages also arise here, that the method is restricted to non-singular mask structures and that the respective partial beam optics can only be of small size.